This invention relates to tool inserts.
Polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PCBN) are hard materials used extensively for producing a variety of cutting and abrading tool inserts. PCD is frequently used in a form in which it is intimately bonded to a cemented carbide substrate during the manufacturing process. PCD and PCBN are produced using manufacturing conditions at which diamond and CBN are crystallographically stable.
One method for using cutting tool materials, for example polycrystalline diamond (PCD) or polycrystalline cubic boron nitride (PCBN), is to wire EDM or laser cut ISO-style cutting inserts from a large disc. Smaller cut pieces, for example a 60 degree triangle of 3 mm side length, are generally brazed into a preformed pocket in a tungsten carbide ‘blank’ or substrate in order to produce a ‘braze-tipped’ insert. The brazed assembly is then ground to produce a finished cutting tool. Many of the substrates used for insert manufacture have a centrally located hole to facilitate screw clamping of the insert in the tool holder—the optimum manner in which to clamp a cutting insert. Inserts with a clamping or ‘pin-locking’ hole may also be clamped using a top-clamp or a top-and-hole-clamp and are thus more versatile than solid or full-top inserts. Braze-tipped inserts with a clamping hole are no more expensive to manufacture than braze-tipped inserts without a clamping hole, due to near-net shape sintering of the holed and pocketed carbide blanks. In the case of larger ‘full-top’ pieces, these are cut to a near net ISO shape dimension, typically a 60-degree triangle with an 11 mm side length. Here, no brazing is required as the blank effectively is the complete finished tool assembly. In most cases this style of tool can only be readily clamped by top clamp arrangement only.
The brazing of small tips in many cases is an expensive and complex process, invariably being manual in nature. From an application point of view, brazing adds no value to a finished tool: In certain applications, the presence of a braze joint can limit the performance of the tool in that excessively large depths of cut and/or feed and excessively high cutting speed, result in melting of the braze joint and catastrophic tool failure (despite the mechanical loads being well below the inherent strength of the cutting tool material). Full-top inserts do not contain a braze joint—the layer of the cutting tool material is intimately bonded to a carbide substrate in the sintering process—thus facilitating maximum utilisation of the cutting tool materials' mechanical properties. In addition, the use of a larger full top piece provides multiple corners that can be used in the tool, for example triangle with 3 cutting corners and square with 4 cutting corners. Therefore in many cases, if there is no restriction on the flexibility of clamping and costs of material remain similar, then a full top insert is preferable to a brazed small tip insert.
In many machining applications where insert-style tooling is used, screw clamping or hole clamping is preferred as it gives better location and rigidity—both known to affect tool performance significantly. In addition, there is a definite and growing trend to machine at higher cutting speeds, which puts a greater requirement on location and rigidity for safety and performance reasons. This is particularly true for rotating tools, such as are applied in milling cutters.
Furthermore, top clamping on the surface of a hard brittle material like PCBN presents an unfavourable loading condition and can lead to or exacerbate insert breakage. Hole clamping and screw clamping apply contact stresses in a more favourable mode compared to top clamping of brittle PCBN material surface. Also, an increasing trend to polish the surface of PCBN inserts makes friction or grip with a top clamping arrangement less secure.
The holing of individual inserts involves complex positioning and machining with laser, EDM and mechanical grinding. This complexity adds to the cost of producing inserts with pin-locking holes.